Recently, a fuel feed rate of an automobile engine (internal combustion engine) is generally controlled based on an intake air flow rate. An intake air flow meter is required for this purpose. Of various types of intake air flow meters, thermal type flow meters have come into widespread use because of their capability to directly sense mass air flow rate.
Of various types of thermal type air flow meters, especially, those using a sensor element (measuring element) manufactured by semiconductor micromachining technology are advantageous in terms of the following: cost reduction, low-power driving, and high-speed responsibility. Therefore, thermal type air flow meters using a sensor element based on this semiconductor technology have become part of mainstream through the years.
Air flow meters constructed as described below are known as conventional art: the substrate of their sensor element is provided with a plurality of diaphragms (thin membrane portions). A heating resistor, an upstream thermal sensitive resistor to the heating resistor and a downstream thermal sensitive resistor thereto served as a flow rate sensor are disposed on these diaphragms. The upstream and downstream sensitive resistors are disposed adjacently to the heating resistor. (Refer to Japanese Patent Laid-Open No. 2001-349759, for example.)
In this thermal type air flow meter according to prior art, a plurality of diaphragms are provided in line in the direction orthogonal to the direction of flow of a fluid to be measured. Such a configuration of diaphragms makes it possible to enhance the strength of the diaphragms and ensure sensitivity and responsibility of the sensor.
The thermal type air flow meter has the following problem: a measuring error is caused due to a change in intake air temperature. Thermal type air flow meters constructed as described below to cope with this are also known as conventional art: an intake air temperature sensor is installed at part of the sensor element of a thermal type air flow meter, and the intake air temperature is measured with it to correct an air flow rate measurement. Thus, measuring errors due to a change in intake air temperature are reduced so that the air flow rate can be measured with higher accuracy.
In this case, the intake air temperature sensor must be separately installed. This increases the number of parts of the thermal type air flow meter, and further requires mounting structure and wiring for installing the intake air temperature sensor. This results in increased production cost.
A technology to integrate an intake air temperature sensor with the sensor element of a thermal type air flow meter has solved this problem. Namely with this technology, the number of parts and wiring for electrically connecting the intake air temperature sensor can be reduced.
Furthermore, the intake air temperature sensor can be integrally provided on the sensor element of the thermal type air flow meter by the following means: the sensor element is formed on a semiconductor substrate of single crystal silicon using micromachining technology. In addition, a thermistor, a temperature sensitive resistor, and the like are formed on the sensor element. As a result, the cost can be further reduced.
Description will be given to an example of a thermal type air flow meter according to conventional art in which a sensor element is provided with an intake air temperature sensor. FIG. 20 is a plan view illustrating conventional art; FIG. 21 is a sectional view taken along the line A-A of FIG. 20; and FIG. 22 is a sectional view taken along the line B-B of the same. In these drawings, numeral 1 denotes a sensor element.
Numeral 2 denotes a semiconductor substrate; 3 and 4 denote diaphragm sections; 5 denotes a heating resistor; 6a and 6b denote thermal sensitive resistors; 7 and 8 denote cavity portions; 9 denotes a temperature compensation resistor; 10 denotes an electrical insulating film; 11 denotes a resistor for intake air temperature sensor; and 12a to 12j denote terminal electrode sections.
Arrow f indicates the direction in which a fluid to be measured (intake air) flows relative to the sensor element 1. In these drawings, therefore, the left side is upstream and the right side is downstream.
As illustrated in FIG. 21 and FIG. 22, the cavity portions 7 and 8 are formed at the semiconductor substrate 2 from one side. The electrical insulating film 10 is formed on the other side of the semiconductor substrate so that these cavity portions 7 and 8 are closed therewith. Thus, the diaphragm sections 3 and 4 are formed. The heating resistor 5 and the thermal sensitive resistors 6a and 6b are disposed on the diaphragm section 3. The resistor 11 for intake air temperature sensor is disposed on the diaphragm section 4.
The thermal sensitive resistor 6a is disposed upstream from the heating resistor 5 adjacently thereto. Downstream from the heating resistor 5, the thermal sensitive resistor 6b is similarly disposed adjacently thereto.
In order to measure the air flow rate with this sensor element 1, the heating resistor 5 is energized to electrically heat itself. At this time, the temperature of the heating resistor 5 is controlled so that it is higher by a certain temperature than the temperature of the temperature compensation resistor 9, that is, the temperature of the fluid to be measured. (The temperature of the fluid to be measured≅the temperature of the semiconductor substrate 2.)
In this state, a change in temperature difference is measured between a pair of the thermal sensitive resistors 6a and 6b disposed upstream and downstream from the heating resistor 5. When assuming that air is not flowing now, the distribution of temperature on the diaphragm section 3 is symmetric between the upstream side and the downstream side with respect to the heating resistor 5. As a result, the thermal sensitive resistor 6a and the thermal sensitive resistor 6b become identical with each other in temperature; therefore, no difference is produced in resistance value.
On the other hand, when air flows in the direction of arrow f, the upstream thermal sensitive resistor 6a is exposed to the air flow. Therefore, it is much cooled, and its temperature falls. Meanwhile, the downstream thermal sensitive resistor 6b is exposed to air heated by the heating resistor 5. Therefore, it is not so much cooled, and its temperature does not fall so much.
As a result, a temperature difference corresponding to the air flow rate is produced between the thermal sensitive resistor 6a and the thermal sensitive resistor 6b, and a difference corresponding to the air flow rate is also produced in resistance value. Consequently, this difference in resistance value is sensed to determine the air flow rate.
The sensor element is configured so that the intake air temperature is measured with the resistor 11 for intake air temperature sensor. For this purpose, the resistor 11 is formed of a resistor material having a large temperature coefficient at zeroth order of the resistance. Therefore, the intake air temperature can be determined by variation in the resistance value of the resistor 11.
As illustrated in FIG. 22, the resistor 11 for intake air temperature sensor is placed on the diaphragm section 4 formed by covering the cavity portion 8 with the electrical insulating film 10. Thus, the thermal capacity is reduced, and the responsibility to the change in intake air temperature is enhanced.
The pressure in the intake pipe of an engine can be steeply increased due to the occurrence of back fire or the like. In such a case, a pressure almost two times greater than the atmospheric pressure can be applied to the sensor element of the thermal type air flow meter, and it can be subject to great mechanical stress. The sensor elements are required to be free from breakage for a long time even under such circumstances.
A defective, for example, a sensor element having damage to its diaphragm section, can exist in a manufacturing process. If such a defective is mounted on an engine and the engine is started, the engine prematurely gets out of order. This is because the sensor element does not have durability enough to endure the harsh pressure conditions mentioned above.
Therefore, failure in the diaphragm sections of a thermal type air flow meter is one of significant causes of such a premature failure. For this reason, defectives must be screened out in the process of manufacture, and screening is a common practice for removing defectives. In case of the sensor element of the thermal type air flow meter, screening is carried out by exerting a predetermined stress on the diaphragm by applying pressure to its diaphragm sections from one side.
For example, the following measures are taken in the semiconductor micromachining process: in the stage of wafer prior to dicing, the cavity portions in positions where the diaphragm sections of the sensor element are formed are sealed. The sensor element is placed in high-pressure environmental test equipment. A pressure difference is produced between one side and the other side of each diaphragm section, and stress is applied to the diaphragm sections. In case of a defective having damage, its diaphragm sections are destroyed.
With this method, a large amount of sensor elements can be tested and screened at a time, and efficient screening can be carried out with ease. The method is effective in providing sensor elements free from defect.
The above-mentioned conventional art does not give consideration to problems that may occur when a temperature sensor for measuring the intake air temperature is formed on the sensor element of a thermal type flow meter. It has trouble with provision of sensor elements free from defect.
As described with respect to the above conventional art, a problem arises when a resistor 11 for intake air temperature sensor is formed on a sensor element 1. Two diaphragm sections exist in the sensor element 1: a diaphragm section 3 in which a heating resistor 5 and thermal sensitive resistors 6a and 6b are formed; and a diaphragm section 4 in which the resistor 11 is formed.
Thus, the test pressure must be varied from one diaphragm section to another diaphragm section during screening because the two diaphragm sections are different in size from each other in the conventional art. Since the sensor element is as small as several millimeters, it is substantially impossible to adjust the test pressure from one diaphragm section to another diaphragm section. As a result, satisfactory screening cannot be carried out.
If a test pressure most suitable for either diaphragm section is selected for screening, the other diaphragm section is exposed to excessive pressure, and its life is shortened. Or, the pressure is insufficient for the other diaphragm section, and satisfactory screening cannot be carried out. In either case, provision of sensor elements free from defect cannot be expected.
An object of the present invention is to provide a thermal type air flow meter wherein whether a sensor element is non-defective or defective can be determined with ease and accuracy by screening.